Electron-optical image converter tube



Oct. 19, 1954 P. SCHAGEN ETAL 2,692,341

ELEcTRoN-OPTICAL IMAGE CONVERTER TUBE Filed Feb. 24, 195o HSE/V7' Patented Oct. 19, 1954 ELECTRN-OPTICAL IMAGE CONVERTER TUBE Pieter Schagen. and Hajo Bruining, Eindhoven, Netherlands, assignors to Hartford' National Bank and rirust Company, Hartford, Conn., as

trustee Application February 24, 1950, Serial-No. 146,052

Claims priority, application Netherlands March 7, 1949 6 Claims.

This invention relates to electron-opticalimage converter tubes for converting an electron image into a visible image. An electron image may, for instance, be obtained by photo-electric emission, when an optical image is projected on to the light-sensitive surface of a photo-electric cathode. Alternatively, it is possible to project an image formed by invisible rays on to a photo-electric cathode sensitive to these rays.

Itis known to cause the photo-electrons so produced to travel, under the action of an electric field to a projection screen spacedI apart from the photo-electric cathode, the photo-electrons striking the projection screeny being distributed to correspond tothe intensity distributionV in the original image. By coating the projection screen with a suitable fluorescent material, an optical image is obtained which is a reproduction of the original image projected onto the photo-electric cathode. If desired, this reproduction maybe enlarged or reduced with respect to the original image and it may have a. greater intensity.

It has, however, been found that this known arrangement involves distortion. of the image reproduced. A-ttempts have been made to avoid this drawback by subjecting the electrons travelling from the cathode to the projectionscreen to the action of one or moreV electron-lenses.. These may be. constituted by electricJ or magnetic fields promoting, on account. of their focussingaction onV the electron beam, a better conformity between the reproduction on the projection screen and the original image., A limitation to the use of one or more electron lensesv is that the device is complicated and, since controllable voltages or currents remaining exactly constant at the ad.- justed value are vrequired for maintaining the lensl elds, special precautions must be taken in designing the supply apparatus..

The present invention has for its objectY to provide an electron optical system suitable for use in an electron-optical image converter which system, without using supplementary electron-lenses andy with the use of a single electric voltage for accelerating the.4 electrons, which voltage need not be exactly constant, produces an image Well resembling the image projected onto the photoelectric cathode.

According to the invention, an electron-optical system comprising a hemispherical concave photo-electric cathode and, spaced therefrom, an anode having an aperture for theY passage of electrons travelling from the photo-electric cathode to a collecting screen arranged behind the anode in aeldless space, is characterized in that the wall of the tube in a vacuum-tight mannerA 2. the surface of the anode facing the photo-electric cathode is convex hemispherical and has a radius of curvature equal to or' smaller than that of the photo-electric cathode less the distance of theanodefrom the photo-electric cathode, and the space between the photo-electric cathode and the anode is surrounded by a conductivev surface electrically connected tothe photo-electric cathode and' hasv a shape such that the action of the electric eld between the photo-electric cathode and the anode is approximately the same as with a concentric arrangement of the anode With respect to the photo-electric` cathode, in- Which arrangement the said conductive' surface is an eX- tension of the hemisphericalcathode surface.

Thequality' of the image may be further improved if the aperture in the anodeY has a constant section over a distance at least equal toits diameter.

The invention is of particular importance in relation to electron-optical image converter tubes which are of a small sizey and the construction of which should thereforeY be as simple as possible. It may, however, alsov be used in other tubes for example in television transmitting tubes with electron-optical increase: inl size of the image toVv betransmitted.

Inorder that theinvention maybe. more clearly understood. and readily carried into effect, it will now be described more fully with reference to the accompanying diagrammatic drawing,l in which one embodiment of an electron-optical image converter tube comprising an electron-optical system according thereto is shown by way of example.

The tube shown in the drawing comprises a closed glass vessel i which consists of a substantially cylindricalA part closed by a hemispherical part 2 at one end. The hemispherical part is coated internally with aV thin' translucent conductive` coating 3'- which may, for example', be obtained by cathodesputtering or by electricdeposition. One supply conductor l is passed through and is electrically connected to thel conductivelayer 3. Onl the layer 3 a photo-electric cathode 5V is formed. The conductive layer 3 may: consist of silver, andthe photo-electric cathode of a layerv of a mixture of silver particles and caesium oxide particles and caesium on which layer a thin caesiu-m layer is absorbed.

At its endI remote from the hemispherical part 2, the tube has a re-entrant partV E` closed by a flat plate on which a fluorescent surfacel or collectingv screen t? is provided. The latter consists 3 of a thin layer of a material which fluoresces on being struck by electrons and may be provided on a thin transparent conductive layer.

To the re-entrant part is connected a metal electrode 9 serving as an anode which consists of a cylindrical tube furnished with a hemispherical cap I0. The latter is provided with a small aperture II for the passage of electrons travelling to the fluorescent screen under the action of a potential difference. This potential may be supplied by a voltage supply (not shown) the negative terminal of which is connected to the conductor 4 and the positive terminal to a conductor I2 which is connected electrically to the electrode 9 and the fluorescent screen 8.

A satisfactory connection of the electrode 9 to the fluorescent screen is provided by a conductive metallic layer I6 on a portion I5 of the re-entrant part of the tube intermediate the electrode 9 and the screen 8 of the re-entrant part. Owing to the shapes of the cathode surface and the end of the electrode 9, an accelerating field is produced between the cathode and the electrode S, which is suitable for obtaining a good reproduction on the fluorescent screen. In order to reproduce a surface of a satisfactory size it is desirable that the ratio between the radii of the anode cap Il] and the photo-electric cathode should not exceed 1:4 with a concentric arrangement. An increase of the radius of curvature of the photo-electric cathode involves a reduction of the surface reproduced, which may be counteracted by increasing the spacing of the two electrodes or by altering the shape of that part of the wall which surrounds the space between the anode cap lil and the cathode. By giving this portion a greater curvature, the field set up between the electrodes is altered such that a stronger concentration of the electron paths is obtained. This construction has the advantage that in spite of a necessarily great radius of curvature of the photo-electric cathode the size of the electron-optical system may be small.

A suitable choice of the radii of the photo-electric cathode and the anode cap I and their relative spacing permits a reproduction with a desired enlargement or reduction to be obtained on the iuorescent screen. However, a certain imagefield curvature still occurs so that with the use of a flat uorescent screen the denition of the reproduction decreases towards the edges. Experiments have revealed that this does not occur if the aperture of the electrode 9 has a constant section over a distance approximately corresponding to its diameter. By providing a block of metal I3 having an aperture I4 this condition is satisfied in the present construction.

What we claim is:

1. An image-transformation tube comprising an envelope, a photo-electric cathode source of electrons having a concave surface with a given radius of curvature positioned at one extremity of said envelope, a collecting screen in a substantially eld-free region in said envelope and remote from said cathode and in the path of said electrons, an anode electrode interposed between said cathode and said collecting screen at a given distance from said cathode and having an aperture therethrough for the passage of electrons from said cathode to said collecting screen, said anode having a convex hemispherical surface facing said concave cathode, the given radius of curvature of said cathode being at least equal to the sum of said given distance and the radius of curvature of said anode hemispherical surface, and a conductive layer in electrical connection with said cathode and surrounding the space between said cathode and said anode for producing a focusing field between said cathode and said anode.

2. An image-transformation tube comprising Yan envelope having a hemispherical portion at one extremity thereof, a photo-electric cathode source of electrons having a concave surface with a given radius of curvature adjacent said hemispherical portion, a collecting screen in a substantially field-free region in said envelope and remote from said cathode and inthe path of said electrons, an anode electrode interposed between said cathode and said collecting screen at a given distance from said cathode and having an aperture therethrough for the passage of electrons from said cathode to said collecting screen, said anode having a convex hemispherical surface facing said concave cathode, the radius of curvature of said anode being no greater than approximately 1A, the given radius of curvature of said cathode, a conductive layer surrounding the space between said cathode and said hemispherical portion of said envelope and in electrical connection with said cathode and extending on said envelope from said cathode towards said anode for producing a focusing field between said cathode and said anode.

3. An mage-transformation tube comprising an envelope, a photo-electric cathode source of electrons having a concave surface and a given radius of curvature positioned at one extremity of said envelope, a collecting screen in a substantially field-free region in said envelope and remote from said cathode and in the path of said electrons, an anode electrode interposed between said cathode and said collecting screen at a given distance from said cathode and having an aperture therethrough of given diameter and constant section over a length approximately equivalent to the said aperture diameter for the passage of electrons from said cathode to said collecting screen, said anode having a convex hemispherical surface facing said concave cathode and being substantially spherically concentric therewith,

a conductive layer in electrical connection with said cathode and surrounding the space between said cathode and said anode for producing a. focusing eld between said cathode and said anode.

4. An image-transformation tube comprising an envelope, a photo-electric cathode source of electrons having a concave surface at one end of said envelope, an anode electrode disposed adjacent said cathode and having a convex hemispherical surface facing said concave cathode and spaced therefrom a given distance, the convex surface of said anode having a given radius of curvature, the concave surface of said cathode having a radius of curvature at least equal to the sum of said given radius and said given distance, said anode having an aperture therethrough for the passage of electrons from said cathode, a conductive layer electrically connected to said cathode and surrounding the space between said cathode and anode for producing Y a focussing fieldtherein, and a collecting screen disposed at the other end of said tube on Vthe side of said anode remote from said cathode for receiving the electrons emanating from said cathode and passing through theaperture in said anode, said collecting screen being disposed in a substantially field-free region in said tube.

5. An image-transformation tube comprising an envelope, a photo-electric cathode source of electrons having a concave surface at one end of said envelope, an anode electrode disposed adjacent said cathode and having a convex hemispherical surface facing said concave cathode and spaced therefrom a given distance, the convex surface of said anode being substantially spherically concentric with the concave surface of said cathode, said anode having an aperture therethrough for the passage of electrons from said cathode, a conductive layer electrically connected to said cathode and surrounding the space between said cathode and anode for producing a focussing field therein, and a fiat luminescent screen disposed at the other end of said tube on the side of said anode remote from said cathode for receiving the electrons emanating from said cathode and passing through the aperture in said anode, said luminescent screen being disposed in a substantially held-free region in said tube.

6. An image-transformation tube comprising an envelope, a photo-electric cathode source of electrons having a concave surface at one end of said envelope, an anode electrode disposed adjacent said cathode and having a convex hemispherical surface facing said concave cathode and spaced therefrom a given distance, the convex surface of said anode being substantially spherically concentric with the concave surface of said cathode and having a radius of curvature not greater than approximately one-quarter the radius of curvature of said cathode, said anode having an aperture therethrough for the passage of electrons from said cathode, -a conductive layer electrically connected to said cathode and surrounding the space between said cathode and anode, means for applying a given potential to said cathode and a greater potential to said anode to thereby produce a focussing field therebetween, and a fiat luminescent screen disposed at the other end of said tube on the side of said anode remote from said cathode for receiving the electrons emanating from said cathode and passing through the aperture in said anode, said luminescent screen being electrically connected to said anode and being disposed in a substantially field-free region in said tube.

References Cited in the le of this patent UNITED STATES PATENTS Number Name Date 2,179,083 Bruche et al. Nov. 7, 1939 2,189,321 Morton Feb. 6, 1940 2,306,881 Heimann et al Dec. 29, 1942 FOREIGN PATENTS Number Country Date 499,661 Great Britain Jan. 26, 1939 

